The present invention relates to an inspection device which inspects thin-film transistors (TFTs). The present invention particularly relates to an inspection device which inspects the characteristics of TFTs for supplying a current to respective pixels included in a display device.
In recent years, organic electroluminescence displays (organic EL displays), which include organic light emitting diodes (OLEDs) as pixels, have been receiving attention as a display device capable of bright display at low power. For the achievement of efficient volume production of the organic EL display, it is important to reduce the manufacturing costs and to enhance the yield by detecting and removing defectives in as early a manufacturing step as possible.
Conventionally, there have been proposed techniques to detect a defective TFT array for supplying a current to OLEDs (see Patent Documents 1 and2). In these techniques, the characteristic of each TFT is inspected prior to a process of forming OLEDs on a TFT array. Thus, in comparison with a case of detecting a defective after OLEDs are formed on a TFT array, it is possible to avoid wasting the process of forming OLEDs and the materials of the OLEDs and thereby to reduce the manufacturing costs.
For example, FIGS. 17(a) and 17(b) each show processes through which an organic EL display panel with OLEDs is manufactured. As shown in FIG. 17(a), when inspection is performed after a cell process for forming OLEDs on a TFT array, the OLEDs have to be formed on a defective TFT array, which is wasteful. On the other hand, as shown in FIG. 17(b), if inspection can be performed prior to the cell process, it is possible to reduce the wastes involved in the cell process.
More specifically, in the technique in Patent Document 1, a common electrode is connected with each TFT in a TFT array prior to a process of patterning on the TFT array pixel electrodes to be connected with OLEDs. A power supply is then connected with the common electrode, and a current flowing in each TFT is measured. Moreover, in the technique in Patent Document 2, pixel electrodes in a TFT array are energized by dipping the TFT array in an electrolytic liquid, and then a current flowing in each TFT is measured.
[Patent Document 1] Japanese Unexamined Patent Publication No. 2002-108243
[Patent Document 2] Japanese Unexamined Patent Publication No. 2002-72918
However, in the manufacture of an organic EL display by the technique in the above Patent Document 1, a process of stripping the common electrode and patterning the pixel electrodes to be connected with the OLEDs is required after the inspection of the TFT array. That is, it can hardly be said that the inspection is performed after the TFT array is finished. Therefore, it can be thought that a defect may occur after the inspection.
According to the technique in the above Patent Document 2, since it is difficult to accurately measure a voltage applied to each TFT, there is a problem that an error in the inspection result may be large. Moreover, there are some occasions where the TFT array dipped in the electrolyte may suffer damage such as breakage and dissolution.
In addition, both the above techniques adopt a measurement method in which a given voltage is applied between a drain and a source of a TFT and then a drain current is measured. In order to realize this method, it is necessary to apply a desired voltage with a precise voltage value to each of the drain and source. In the inspection prior to the cell process, however, since one terminal of the drain and source is not connected with the circuit, it is difficult to apply a stable voltage to the terminal.